Sound of brook at edge of forest (5)
I believe the answer is:
trill
'sound' is the definition.
(a trill is a quavering sound)
'brook at edge of forest' is the wordplay.
'brook' becomes 'rill' (I've seen this before).
'at' means one lot of letters go next to another (I've seen this in other clues).
'edge of' suggests the final letters (one of the edges of the word).
The final letter of 'forest' is 't'.
'rill' after 't' is 'TRILL'.
'of' is the link.
(Other definitions for trill that I've seen before include "Ornament" , "Sound of rapidly alternating notes" , "High-pitched warbling sound" , "Fast alternation of notes" , "Shrill warble" .)